In pulp production, fibres are caused to be suspended in a water solution by means of mechanical and/or chemical processes. The fibres are also washed at different point along the process. Therefore, in many stages along a pulp production line, there is a need for dewatering the pulp suspension. In a typical example, dewatering of the pulp may be performed from an inlet pulp concentration of 2-10% by weight to an output pulp concentration of 20-50% by weight.
One type of dewatering device is the twin-roll dewatering device. In such a device, the fibre suspension is brought into contact with the mantle plate of a roll. The mantle surface is typically perforated with dewatering channels in a relatively dense pattern. Two such rolls are rotated in opposite directions to bring the fibre suspension into a small gap between the rolls. The water is thereby pressed out from the suspension through the dewatering channels of the mantle plate and into flow paths within the structures of the rolls. The majority of the fibres remain outside the mantle and a pulp with much lower water content exits in the small gap on the other side. The dewatered liquid is typically transported axially along the rolls and exits the device in the vicinity of one or both of the roll ends.
The dewatering efficiency depends to a high degree on the nip between the rolls. In order to achieve a well-defined and efficient dewatering, it is advantageous to have the same load along the axial direction of the rolls within the nip section. However, since the force holding the rolls together is applied at the axial ends of the rolls and the load from the fibre suspension dewatering operation is spread over the entire length of the rolls, there will be a certain tendency of a bending of the rolls. The gap at the middle parts of the rolls will therefore be somewhat larger than the gap in the vicinity of the ends of the rolls. For short rolls, such bending of the rolls may be small enough to be neglected or at least acceptable. However, since the tendency is to provide longer and longer rolls in order to increase the put-through, such bending effects may be quite severe. In the middle part of the device, the dewatering efficiency will be reduced, and if the gap is generally decreased to mitigate such effects, the load at the rolls close to the ends becomes very high indeed, which may cause mechanical damage on the rolls. The high load is caused by a highly non-linear compressibility of the pulp.
Similar effects are also present in e.g. paper machines. The typical approach for compensate for such effects is to perform a machining of the rolls in order to give a somewhat smaller roll radius close to the ends of the rolls and a larger radius in the middle. However, paper machine solutions are not applicable for pulp dewatering purposes, since the dewatering process takes place via a relatively thin mantle plate. The volume inside the mantle plate is used for allowing the liquid extracted from the fibre suspension to exit the device. Machining of a mantle plate of a fibre suspension dewatering roll will either decrease the mechanical strength of the mantle at the roll ends or require longer dewatering channels through the mantle plate in particular at the centre of the rolls, which is not acceptable.